Anaerobic digestion is typically done in a single tank or in two tanks connected in series. The tanks are typically cylindrical and mixed so as to operate as a continuously stirred tank reactor (CSTR). Where two tanks are used, the effluent from the upstream tank flows to the downstream tank. In some cases, two tanks are used to separate the process into an acid phase and a gas or methanogenic phase. In such a phased digestion system, the volume of the acid phase tank is much lower than that of the gas phase tank.